(1) Field of the Invention
The present invention relates to the enhancement of safety in the work place and particularly to the disabling of equipment upon the opening of an access door. More specifically, this invention is directed to electrical interlocks and especially to improvements in safety switch assemblies. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
The interruption of operation of, and particularly the discontinuance of the delivery of electrical power to, a piece of equipment when an access door or protective cover is opened is often necessary. For this purpose, electrical controls, known in the art as interlocks, are commonly provided. Such interlocks will typically include a normally open safety switch which will be closed when the door, gate or other protective cover is in the closed condition and the apparatus associated therewith in operation. The interlock should be automatically and positively opened when the access door or cover is opened.
One type of safety switch assembly known in the prior art comprises a contact bridge, with a pair of outwardly extending rods, mounted on the door and cooperating with a pair of resilient contact members located on the door frame. The bridge establishes a path for current and thus comprises a current carrying switch member. The switch will be positively opened if the door on which the contact bridge is mounted is opened. While this type of safety switch is effective and of uncomplicated construction, it's use is limited to clean and dry environments since the switch is neither dust nor moisture proof.
It is also known to provide safety switches which are mounted on the hinge pin of a protective door or cover. Such hinge pin mounted swithes, however, are comparatively expensive and have limited utility since they may be employed only on doors which have a hinging movement.
Key-operated safety switches are also known in the prior art. In safety switches of the key-operated type, the interlock feature is engaged and disengaged by means of a lock and a code key matched thereto. While such key-operated switches provide positive protection, and thus may be used under dangerous conditions, they have the obvious disadvantage that the key must be available. This often poses a substantial inconvenience and may prevent rapid opening of the protective cover to disable the apparatus in an emergency.
A further type of prior art safety switch, in which the closure of switch contacts located in a housing mounted on the door frame is effected by a hook-shaped member mounted on the door, may be seen from Austrian Patent Specification No. 234,956. In safety switches of the type shown in this Austrian patent the hook-shaped member, which pivots as the door closes, engages a switch cam which, in turn, effects closure of the switch contacts. The hook-shaped member is generally U-shaped in cross-section and the frame mounted switch housing is provided with a corresponding U-shaped opening. A major disadvantage of safety switches of this type is that they may be actuated only from one side, i.e., there is only a single position for the hook-shaped member in which the switch can be operated. This lack of flexibility in mounting is a serious disadvantage since it is frequently necessary or desirable, for example in situations where space is limited, for the door mounted portion of the switch actuating system to approach the switch housing from an angle which is different from the single position of actuation possible with the switch of the aforesaid Austrian patent.